1. Field of Invention
This invention relates generally to the manufacture of semiconductor structures. More particularly, the invention entails the use of strained gettering layers for fabrication of semiconductor structures.
2. Discussion of Related Art
Entrapment of a mobile species, referred to as “gettering,” is ubiquitous to semiconductor processing. A familiar application for gettering is contamination control, wherein contamination is drawn away from critical device regions by confining fast diffusing impurity species to isolated gettering regions in the semiconductor wafer. For this reason, gettering methods are typically engineered into semiconductor substrates during wafer manufacturing. Some of these techniques include SiO2 precipitation, intentional mechanical damage and introduction of internal voids by He+ implantation.
Another application of gettering, albeit inadvertent, occurs during the layer exfoliation technology of SOITEC Corporation of France and marketed as the Smartcut™ process, wherein a thin layer of material is separated from a donor wafer by means of hydrogen and/or helium ion implantation. The combination of wafer bonding and layer exfoliation, also known as layer transfer, is the basis for fabrication of many advanced substrate structures including silicon-on-insulator (SOI), strained-silicon-on-insulator (SSOI) and germanium-on-insulator (GOI). Due to its versatility, layer transfer can be used to transfer virtually any material from a donor wafer onto a second wafer (called the handle wafer) of arbitrary composition for material integration applications.
Layer exfoliation occurs due to the joint action of implanted hydrogen and defects created within the donor wafer by the implantation procedure. These material imperfections, including point defects (e.g., vacancies, interstitials) and extended defects (e.g., platelets, vacancy clusters, voids) behave as gettering centers for hydrogen, thus preventing its effusion during subsequent annealing treatments. Platelets and voids are known to be particularly efficient gettering centers for hydrogen. During annealing, the hydrogen pressurizes such defects, leading to mechanical cleaving below the surface of the wafer and exfoliation of the surface layer. Furthermore, defects also behave as mutual gettering centers. One such example of mutual defect gettering is the coalescence of vacancies to form vacancy clusters, voids and other extended defects, which in turn contribute to the overall exfoliation process. Cavities are particularly strong gettering sites for vacancies, causing the volume of the cavity to expand during post implantation annealing.
Conventional layer exfoliation methods possess a number of limitations. A major drawback of conventional methods is the requirement for a relatively high implantation dose (˜1×1017 cm−2) to induce layer transfer. Although modern ion implantation tools have the capability of delivering such high doses, implantation steps are nonetheless expensive. Another limitation is that processing temperatures must be minimized during bonding of thermally mismatched wafer pairs to avoid bond failure during the annealing stage of layer transfer. Another drawback is that small variations in implantation conditions may have a large effect on the density and type of defects formed during implantation which, in turn, might alter the annealing schedule required for layer transfer. Such variations may lead to process instabilities which must be avoided for commercial application of layer exfoliation.
A number of methods that utilize hydrogen gettering have been proposed in efforts to improve the layer exfoliation process. One technique involves co-implantation of boron with hydrogen in a process, wherein electronically active acceptor states getter hydrogen through formation of H complexes. This gettering effect may prevent effusion of the implanted hydrogen and provide a nucleation site for platelet formation. Co-implantation of boron also increases the amount of damage in the wafer, potentially increasing its hydrogen gettering efficiency, and reducing the thermal budget for layer transfer. Despite this benefit, this approach requires a large implantation dose to induce layer exfoliation, resulting in broad damage profiles and increased processing costs.
Another modification to the Smartcut™ process involves the reduction of the implantation dose for layer exfoliation via the co-implantation of H+ and He+ ions. The process exploits the efficiency of H in producing extended defects with the efficiency of He for pressurizing these defects, ultimately leading to material cleavage. The presence of both hydrogen and helium species has a synergistic effect and allows Si layer transfer with a combined H+/He+ implantation dose of less than 2×1016 cm−2. Although the H+/He+ co-implantation method reduces the overall dose needed for exfoliation, it relies on an implantation procedure to produce the defects that ultimately cause layer transfer. Ion implantation produces a diffuse damage profile, resulting in a broad gettering region for the species participating in layer exfoliation and, therefore, a diffuse cleavage plane with large surface roughness. Furthermore, even more efficient layer transfer would be possible if the species participating in layer exfoliation were confined to a narrower region.
Yet another modification involves the formation of a damaged region below the surface of the donor wafer by means of an inert gas or self ion implantation. This process offers a low cost solution for SOI since the required dose of the implant is only ˜1015 cm−2. However, a final high temperature anneal is required to anneal out the lattice damage caused by the heavy ion implantation step.
Still another variation makes use of a cleave layer, wherein a donor substrate incorporates a compressively strained SiGe cleave layer along which cleaving action is claimed to occur. Implanted H segregates to the periphery of the cleave layer, resulting in a lower H concentration in the cleave layer. This behavior is expected and corresponds with other experimental observations where interstitial hydrogen is repelled from regions of compressive strain. The combination of these effects results in a situation where the location of the various components that participate in layer exfoliation (e.g., H, and the cleave layer) do not coincide.
A need therefore exists for an improved process incorporating a strained gettering layer that is effective for H and/or He, allowing for accumulation of H and/or He within the gettering layer. In addition, a need also exists for an improved process incorporating a strained gettering layer that is effective in accumulating point defects (e.g., interstitials, vacancies) introduced during the implantation process. In this way, components that are vital to layer exfoliation segregate within a confined strained layer, creating a selectively damaged layer along which layer exfoliation occurs.